This invention relates to a method and apparatus for achieving transmit diversity in telecommunication systems and, more particularly, to a method and apparatus for space-time coding signals for transmission on multiple antennas.
As wireless communication systems evolve, wireless system design has become increasingly demanding in relation to equipment and performance requirements. Future wireless systems, which will be third and fourth generation systems compared to the first generation analog and second generation digital systems currently in use, will be required to provide high quality high transmission rate data services in addition to high quality voice services. Concurrent with the system service performance requirements will be equipment design constraints, which will strongly impact the design of mobile terminals. The third and fourth generation wireless mobile terminals will be required to be smaller, lighter, more power-efficient units that are also capable of providing the sophisticated voice and data services required of these future wireless systems.
Time-varying multi-path fading is an effect in wireless systems whereby a transmitted signal propagates along multiple paths to a receiver causing fading of the received signal due to the constructive and destructive summing of the signals at the receiver. Several methods are known for overcoming the effects of multi-path fading, such as time interleaving with error correction coding, implementing frequency diversity by utilizing spread spectrum techniques, or transmitter power control techniques. Each of these techniques, however, has drawbacks in regard to use for third and fourth generation wireless systems. Time interleaving may introduce unnecessary delay, spread spectrum techniques may require large bandwidth allocation to overcome a large coherence bandwidth, and power control techniques may require higher transmitter power than is desirable for sophisticated receiver-to-transmitter feedback techniques that increase mobile terminal complexity. All of these drawbacks have negative impact on achieving the desired characteristics for third and fourth generation mobile terminals.
Antenna diversity is another technique for overcoming the effects of multi-path fading in wireless systems. In diversity reception, two or more physically separated antennas are used to receive a signal, which is then processed through combining and switching to generate a received signal. A drawback of diversity reception is that the physical separation required between antennas may make diversity reception impractical for use on the forward link in the new wireless systems where small mobile terminal size is desired. A second technique for implementing antenna diversity is transmit diversity. In transmit diversity a signal is transmitted from two or more antennas and then processed at the receiver by using maximum likelihood sequence estimator (MLSE) or minimum mean square error (MMSE) techniques. Transmit diversity has more practical application to the forward link in wireless systems in that it is easier to implement multiple antennas in the base station than in the mobile terminal.
Transmit diversity for the case of two antennas is well studied. Alamouti has proposed a method of transmit diversity for two antennas that offers second order diversity for complex valued signals. S. Alamouti, xe2x80x9cA Simple Transmit Diversity Technique for Wireless Communications,xe2x80x9d IEEE Journal on Selected Areas of Communications, pp. 1451-1458, October 1998. The Alamouti method involves simultaneously transmitting two signals from two antennas during a symbol period. During one symbol period, the signal transmitted from a first antenna is denoted by s0 and the signal transmitted from the second antenna is denoted by S1. During the next symbol period, the signal xe2x88x92s1* is transmitted from the first antenna and the signal s0* is transmitted from the second antenna, where * is the complex conjugate operator. The Alamouti method may also be done in space and frequency coding. Instead of two adjacent symbol periods, two orthogonal Walsh codes may be used to realize space-frequency coding.
Extension of the Alamouti method to more than two antennas is not straightforward. Tarokh et al. have proposed a method using rate=xc2xd, and xc2xe SpaceTime Block codes for transmitting on three and four antennas using complex signal constellations. V. Tarokh, H. Jafarkhani, and A. Calderbank, xe2x80x9cSpace-Time Block Codes from Orthogonal Designs,xe2x80x9d IEEE Transactions on Information Theory, pp. 1456-1467, July 1999. This method has a disadvantage in a loss in transmission rate and the fact that the multi-level nature of the ST coded symbols increases the peak-to-average ratio requirement of the transmitted signal and imposes stringent requirements on the linear power amplifier design. Other methods proposed include a rate=1, orthogonal transmit diversity (OTD)+space-time transmit diversity scheme (STTD) four antenna method. L. Jalloul, K. Rohani, K. Kuchi, and J. Chen, xe2x80x9cPerformance Analysis of CDMA Transmit Diversity Methods,xe2x80x9d Proceedings of IEEE Vehicular Technology Conference, Fall 1999, and M. Harrison, K. Kuchi, xe2x80x9cOpen and Closed Loop Transmit Diversity at High Data Rates on 2 and 4 Elements,xe2x80x9d Motorola Contribution to 3GPP-C30-19990817-017. This method requires an outer code and offers second order diversity due to the STTD block (Alamouti block) and a second order interleaving gain from use of the OTD block. The performance of this method depends on the strength of the outer code. Since this method requires an outer code, it is not applicable to uncoded systems. For the case of rate=⅓ convolutional code, the performance of the OTD+STTD method and the Tarokh rate=xc2xe method ST block code methods are about the same.
The present invention presents a method and apparatus for space-time coding signals for transmission on multiple antennas. In the method and apparatus, a received input symbol stream is transformed using a predefined transform and transmitted on a first set of N antennas. The same input symbol stream is then offset in time by M symbol periods to generate an offset input symbol stream. The offset input symbol stream may be offset so as to lead or lag the input symbol stream. The offset input symbol stream is then transformed using the predefined transform and transmitted on a second set of N antennas. A third through Xth set of N antennas may be utilized for transmission by successively offsetting the offset input symbol stream by an additional M symbol periods for each additional set of N antennas used, before performing the transform and transmitting on the additional set of N antennas. The transform may be applied in either the time domain or Walsh code domain.
At the receiver, the transmitted symbols may be recovered using a maximum likelihood sequence estimator (MLSE) decoder implemented with the Viterbi algorithm with a decoding trellis according to the transmitter.
In an embodiment, 4 antennas are used for transmission. Every 2 input symbols in a received input symbol stream are transformed in the time domain by an Alamouti transform and the result is transmitted on antennas 1 and 2 during the time of two symbol periods. The received input symbol stream is also delayed for two symbol periods, and this delayed input symbol stream is input to an Alamouti transform where every two symbols are transformed and the delayed result is transmitted on antennas 3 and 4 during the time of two symbol periods. The transmitted signal may be received and decoded using an MLSE receiver. The method and apparatus provides diversity of order four and outperforms other proposed extensions of the Alamouti method to more than two antennas by approximately xc2xd to 1 dB for uncoded transmissions.
In an alternative embodiment using 4 antennas, every 2 input symbols in a received input symbol stream are transformed in the Walsh code domain. The Alamouti coded symbols are transmitted on two orthogonal Walsh codes, W1 and W2 simultaneously on antennas 1 and 2. Both W1 and W2 span two symbol periods, which maintains the transmission rate at two symbol periods. The received input symbol stream is also delayed for two symbol periods and the Alamouti transform is also applied in the Walsh code domain to the delayed input symbol stream. This delayed result is transmitted on antennas 3 and 4 during the time of two symbol periods.
In a further alternative embodiment using 8 antennas for transmission, a rate=xc2xe ST block code is combined with a 4 symbol delay. Every three symbols in an input symbol stream are transformed by the ST block code and transmitted on antennas 1-4. The received input symbol stream is also delayed for four symbol periods, and this delayed input symbol stream is input to the ST block code transform where every three symbols are transformed and the delayed result is transmitted on antennas 4-8 during the time of four symbol periods.